The present invention relates to a laser weld quality monitoring method and system. In particular, the invention relates to a laser weld quality monitoring method and system adapted to monitor a quality of a YAG laser weld, such as for an occurrence of a porous, under-filled, or non-welded status.
The welding of very thin steel sheets, such as for a vehicle body, is performed by a laser welding. In comparison with a spot welding, the laser welding has many advantages such that it is applicable to a one-side welding without the need of clamping steel sheets from both obverse and reverse, and that it allows an easy welding even at an inside of a complicate narrow groove. However, as a disadvantage, it tends to suffer a degradation of welding quality caused by a failed lapping accuracy between steel sheets or accrued suddenly at a stained welding part.
Therefore, the monitoring of a laser weld is performed by predicting a weld quality in a real-time manner. Japanese Patent Application Laying-Open Publication No. 2000-271768 has disclosed techniques of using a pair of sensors having their detection angles different from each other, for sensing intensities of light from a plume occurring at a keyhole of a weld by a YAG (Yurium Aluminum Garnet) laser and reflection of light of the YAG laser, to detect variations of output, welding, and inter-sheet gap as welding conditions, thereby performing a real-time prediction of a quality of the laser weld.
For the conventional method of monitoring a quality of laser weld, it is possible to detect occurrences of a significant grooved state of weld (hereafter sometimes referred to as xe2x80x9cunder-filledxe2x80x9d state or simply xe2x80x9cunder-fillxe2x80x9d) and a non-conforming state of weld significantly deviated from a specified welding condition (hereafter sometimes referred to as xe2x80x9cnon-conformingxe2x80x9d state or simply xe2x80x9cnon-conformityxe2x80x9d). It however is difficult to detect an occurrence of a significant porous state of weld (hereafter sometimes referred to as xe2x80x9cporousxe2x80x9d state or simply xe2x80x9cporosityxe2x80x9d or xe2x80x9cporexe2x80x9d).
The difficulty in detection of an occurrence of porous state in the conventional weld quality monitoring method resides in that a decision on quality is made of a state of weld based on the intensity of light emitted from a melt (with a xe2x80x9ckeyholexe2x80x9d) irradiated by a laser beam, irrespective of the fact that the porous state is caused by a mixing of zinc vapor inside the keyhole, which mixing seldom imparts significant variations in the intensity of emitted light from the keyhole.
Moreover, in a lap welding, if the inter-sheet gap is too great, there occurs an incomplete welding as a failure of weld between lapped steel sheets (hereafter sometimes referred to as xe2x80x9cnon-weldedxe2x80x9d state or xe2x80x9cfailed lap weldxe2x80x9d), of which detection also is difficult in the conventional weld quality monitoring method.
Furthermore, in the conventional weld quality monitoring method, occurrences of the porous state, weld state such as an under-filled excepting a non-welded, and non-conforming state can be detected in different manners depending on the weld state, so that their detection needs a very complicate calculation process, with a commensurate great burden to be imposed on a CPU (central processing unit) for the calculation process.
Still more, in the conventional weld quality monitoring method which allows a facilitated detection of an occurrence of a porous state, weld state such as an under-filled excepting a non-welded, or non-conforming state over an entire welding region, if the occurrence of such a state is localized merely in part of the welding region, it also is uneasy to detect this state.
The present invention is made with such points in view. It therefore is an object of the invention to provide a laser weld quality monitoring method and system allowing for an ensured detection of occurrences of weld states such as a porous, under-filled, and non-welded, without an undesirable increase of burden imposed on a processing capacity of CPU, as well as for that of localized occurrences of weld states such as a porous, under-filled, and non-welded, in a welding region by a YAG laser.
As a solution to achieve the object, according to an aspect of the present invention, there is provided a laser weld quality monitoring method comprising: welding a part of work with a laser beam irradiated thereon from a YAG laser; detecting a varying intensity of light reflected from the welding part to provide a detection signal; determining a value of signal power of a frequency spectrum in a specified frequency band of the detection signal; and making a decision for a porous state of the welding part to be significant as the value of signal power exceeds a threshold of weld quality, and to be insignificant as the value of signal power does not exceed the threshold of weld quality.
According to another aspect of the present invention, there is provided a laser weld quality monitoring method comprising: irradiating a laser beam from a YAG laser to a welding part of work; detecting light reflected from the welding part; calculating a frequency distribution from a set of data of the detected light within a interval of time; calculating, from the frequency distribution, a first signal power sum in one of a first frequency band for detecting an under-filled state and a second frequency band for detecting a porous state, and a second signal power sum in a third frequency band for detecting a non-welded state; mapping a combination of calculated values of the first and second signal power sums, in a region defined by a combination of a first axis representing the first signal power sum and a second axis representing the second signal power sum, including a sub-region representing a non-conforming state as one of the under-filled state, the porous state, and the non-welded state; and making a decision for the welding part to have the non-conforming state, as the combination of calculated values is mapped in the sub-region.
Further, to achieve the object described, according to another aspect of the present invention, there is provided a laser weld quality monitoring system comprising: a welder configured to weld a part of work with a laser beam irradiated thereon from a YAG laser; a detector configured to detect a varying intensity of light reflected from the welding part to provide a detection signal; a value determiner configured to determine a value of signal power of a frequency spectrum in a specified frequency band of the detection signal; and a decision-maker configured to make a decision for a porous state of the welding part to be significant as the value of signal power exceeds a threshold, and to be insignificant as the value of signal power does not exceed the threshold.
According to another aspect of the present invention, there is provided a laser weld quality monitoring system comprising: a laser welder configured to irradiate a laser beam from a YAG laser to a welding part of work; a detector configured to detect light reflected from the welding part; a calculator configured to calculate a frequency distribution from a set of data of the detected light within a interval of time; a calculator configured to calculate, from the frequency distribution, a first signal power sum in one of a first frequency band for detecting an under-filled state and a second frequency band for detecting a porous state, and a second signal power sum in a third frequency band for detecting a non-welded state; an operator configured to map a combination of calculated values of the first and second signal power sums, in a region defined by a combination of a first axis representing the first signal power sum and a second axis representing the second signal power sum, including a sub-region representing a non-conforming state as one of the under-filled state, the porous state, and the non-welded state; and a decision-maker configured to make a decision for the welding part to have, the non-conforming state, as the combination of calculated values is mapped in the sub-region.